In typical automotive power steering gears, a threshold manual effort level required to initiate power assist and the relationship between power assist and manual steering effort above the threshold effort are determined by a resilient element such as a torsion bar disposed between an input shaft of the steering gear and an output shaft of the steering gear. When a driver turns a steering wheel connected to the input shaft, the torsion bar twists and relative angular displacement is effected between a valve spool connected to the input shaft and a valve sleeve connected to the output shaft. The relative movement of the valve elements ports pump boost pressure to a steering assist fluid motor. With these typical power steering gears, comfortable low speed steering is assured because the spring rate of the torsion bar is relatively low and because power assist increases exponentially after the threshold effort level is achieved.
Power steering gears have been proposed in which the threshold manual effort increases with increasing vehicle speed. For example, U.S. Pat. No. 4,765,427, issued Aug. 23, 1988 and assigned to the assignee of this invention, describes a detent reaction power steering gear wherein a detent element on the output shaft penetrates a detent groove in the input shaft with a force which depends on a fluid detent pressure behind the detent element. A transmission driven pump provides detent pressure proportional to vehicle speed so that the detent elements more forcefully resist the onset of relative angular displacement between the input and output shafts and, consequently, between the valve elements as vehicle speed increases.
Power steering gears have also been proposed in which the relationship between power assist and manual effort is more linear. For example, U.S. Pat. No. 4,768,604, issued Sept. 6, 1988 and assigned to the assignee of this invention, describes a steering system wherein high pressure fluid is bypassed from the high pressure side of a steering assist fluid motor to the low pressure side to reduce assist.
A power steering system according to this invention includes a relatively simple power steering gear having detent reaction and a control system for the detent reaction whereby a comfortably low threshold manual effort level is achieved at low speed, threshold manual effort increases with increasing vehicle speed above low speed, and the relationship between power assist and manual effort above the threshold effort is more linear.